Diesel engine emissions include toxic particulate matter carried with engine exhaust. The amount of particulate matter expelled from a diesel engine is regulated by environmental regulations. Many treatment systems for diesel engines, as are well known in the art, provide for removal of the particulate matter from the diesel engine emissions. Aftertreatment systems generally include a diesel particulate filter (DPF), a diesel oxidation catalyst (DOC), a selective catalytic reduction (SCR) catalyst, and an ammonia oxidation (AMOx) catalyst, all placed in the exhaust gas stream of a diesel engine. The DPF captures and collects particulate matter (PM) from the engine exhaust. The DOC catalyst oxidizes nitrogen oxide (NO) to nitrogen dioxide (NO2). The SCR catalyst is used to convert oxides of nitrogen (NOx) and ammonia (NH3) to nitrogen gas (N2) and water (H2O). However, if excess NH3 is introduced into the exhaust stream upstream of the SCR catalyst, “ammonia slip” may occur, resulting in some NH3 being carried out with the exhaust, if not all the NH3 is converted by the SCR catalyst. The AMOx catalyst helps to reduce ammonia slip by converting the excess NH3 to N2 and H2O.
The AMOx catalyst may age and degrade over time. An aged catalyst may have reduced operability and may not convert all of the excess NH3 to N2. This may contribute to ammonia slip, resulting in NH3 released into the atmosphere with the exhaust. NH3 may be hazardous when released in the atmosphere and may be caustic to other materials.
An exemplary treatment system is described in U.S. Pat. No. 7,722,845 that issued to Caudle et al. on May 25, 2010 (the '845 patent). The system of the '845 patent comprises an AMOx catalyst containing platinum to efficiently convert NH3. However, the system of the '845 patent is not configured to impede the various operational factors that contribute to AMOx catalyst aging. Therefore, the useful life of the AMOx catalyst described in the '845 patent may be relatively short compared to the other components of the treatment system. As a result, the system of the '845 patent may fail to convert all the NH3 when the catalyst has aged.
The disclosed system is directed to overcoming one or more of the problems set forth above and/or other problems of the prior art.